The term “guiding device” is understood to mean any device intended to control the propagation of electromagnetic waves. These devices cover in particular: waveguides, electromagnetic cavities, reflectors, diffusers, antennas, filters and attenuators.
Some of these guiding devices are used not only to control the propagation of electromagnetic waves, but they may also employ electron beams or beams of other particles that may or may not be provided with an electric charge. This is the case in particular for all electron tubes and nearly all particle accelerators.
In the rest of this text, for more succinct expression, and to differ from the usually accepted meaning of the term “waveguide”, we will simply call any guiding device within the meaning defined above a “waveguide”.
One particular example of a waveguide within our intended meaning is that of cavities for high-precision atomic clocks. In this example, the cavity consists of a single body, of complex shape, which includes several holes.
FIGS. 1a and 1b show one particular example of a cavity employed for producing an atomic clock. A microwave is introduced via an access port 4. This microwave interacts with a cesium beam (Jc) that passes through the cavity and is introduced via an aperture 6.
In all waveguides, the waves are confined by the positioning, in space, of physical objects called “bodies”. Like any physical object, a body occupies a volume that is bounded by one or more closed surfaces. The vicinity of such a closed surface is called the “wall” of the body.
The particular feature of the body of a waveguide is that at least part of the surface of its walls interacts directly with the guided or confined electromagnetic waves and consequently must be endowed with controlled electromagnetic properties.
That part of a wall which interacts directly with the guided or confined electromagnetic waves, and which must be endowed with controlled electromagnetic properties, is called the “active” part of the wall. In the rest of the description, the term “active wall” will refer to an “active” part of a wall of a waveguide body.
It is the geometric and electromagnetic properties of the active walls that determine the electromagnetic properties of the waveguide.
Two types of characteristics of these active walls directly determine the electromagnetic behavior of the waveguide:
(1) their geometric shape; and
(2) their reflectivity with respect to electromagnetic waves.
In the most demanding applications, the aim is to achieve very precise control of the electromagnetic wave propagation, which means that the geometric shape of the active walls of the waveguide must be controlled very precisely.
Depending on the application, the aim is to have different reflectivities on the active walls.
For example, for an attenuator, the aim is to absorb the waves in the active wall.
However, for most applications, in particular for a waveguide in the usual meaning of the term, for an electromagnetic cavity or for a reflector, the aim is usually for the active wall to be as reflective as possible with respect to the waves, without absorbing the energy of the wave. This means that the electrical conductivity of the body near the wall must be as high as possible at the frequencies corresponding to the waves present in the waveguide in operation.
More precisely, for these types of waveguide, which will be called “low-absorption” waveguides, it is necessary to ensure that the conducting material constituting the active wall, in direct contact with the electromagnetic waves, has the optimum electrical conductivity over a thickness equal to a few “skin depths” of the most penetrating components (with respect to the walls) of the wave that should reside in or travel through the waveguide.
For example, for a waveguide intended to be used at ambient temperature and at frequencies close to 10 GHz, the walls of the waveguide being made of copper, the skin depth is a fraction of one micron and it is sufficient for there to be less than 10 microns of copper on the wall in order to approach to better than 99% the quality factor of a cavity made of solid copper.
In specific waveguide applications, the main functionality of controlling the electromagnetic wave propagation is not the only one involved in the specification and design of the waveguide. Many other contingencies must also be considered.
The most common additional criteria relate to the following points:                the volume and total mass of the waveguide;        its resistance to mechanical attack, particularly accelerations, vibrations, impacts and stresses;        its resistance to thermal attack, particularly temperature rises during heat treatments and temperature cycling during operation;        its resistance to chemical attack, particularly to corrosive atmospheres;        the electrical conductivity of the volume or certain regions of the inactive walls of the bodies;        the manufacturability and manufacturing cost of the waveguide;        its functional endurance in the intended application environment; and        its ability to discharge the dissipated heat, very often essentially in the active walls.        